Method for manufacturing quartz glass crucible

ABSTRACT

Present invention suppresses undesirable effects of the bubbles trapped in a silica glass crucible on single crystallization during the pulling process under a high-temperature load. When raw material powder is melted in a mold  1,  graphite components of electrodes and impurities contained in the raw material are removed by introducing hydrogen gas and/or oxygen gas immediately after the start of arc discharge. Graphite and impurities are prevented from entering the product crucible, thereby suppressing the volume increase rate of the bubbles and reducing the inner pressure of the bubbles. Gases remaining in voids of an accumulated layer of silica powder formed inside the mold  1  can be replaced with helium gas, by supplying helium gas to the accumulated layer from the mold  1.

TECHNICAL FIELD

The present invention relates to a method of producing a silica glasscrucible, and especially to a method of producing a silica glasscrucible which can improve the yield of semiconductor single crystalwhen used for pulling of semiconductor single crystal.

BACKGROUND

As a method of producing semiconductor single crystal such as siliconsingle crystal, “the pulling method” (Czochralski method or CZ method),in which seed crystals as cores are immersed in a liquid surface ofmolten semiconductor materials such that single crystal is grown fromthe seed crystal, is known. A silica glass crucible is employed formelting semiconductor materials. In recent years, in order to reduce theproduction cost in the pulling process of single crystal, Multi-pullingmethod or large-diameter silicon single crystal pulling method has beenattempted. However, in the case of such new methods, a silica glasscrucible of a large inside or inner diameter are often required.

When the inner diameter of the silica glass crucible is increased and alarge amount of semiconductor materials are melted in the silica glasscrucible, the melting time becomes longer and thus a longer time isrequired for entire pulling process. In order to shorten the timerequited for the pulling process, increasing the amount of heat (theamount of inputted heat) which is given to the silica glass crucible bya heater is one possible solution. Further, it is preferable that theamount of inputted heat is relatively large for maintaining thetemperature of a large amount of the liquid of the semiconductormaterials at a predetermined temperature.

However, when the amount of the inputted heat is large, the followingundesirable outcomes may occur. It is known that a considerable amountof gaseous components is mixed into the silica glass crucible from theair during the production process and remains as bubbles therein. Thesebubbles tend to expand when the silica glass crucible is used at a hightemperature, and the bubbles present in the transparent layer as theinner surface layer of the crucible, in particular, tend to explode as aresult of increase in volume thereof. Pieces of silica glass resultedfrom such explosions are mixed into the liquid of silicon, transferredin the melt liquid as cristobalite by convection, and may be depositedto the lower end of single crystal of silicon which are in the midst ofthe pulling process. The single crystal may then collapse from theportion at which the silica glass piece is deposited, therebydeteriorating the yield of the semiconductor single crystal. Thisundesirable phenomenon becomes more significant when the amount ofinputted heat is increased or the heat load is increased as the timerequired for the pulling process is prolonged, due to use of the silicaglass crucible of a large inside diameter. The larger the number or thesize of the bubbles present in the transparent layer is, or the largerthe volume increasing rate of the bubbles during the single crystalpulling process is, the more easily the bubbles explode.

As a method of producing silica glass which has a highly transparentglass layer having a relatively small amount of bubbles, there exists aknown method in which silica glass is produced by melting silica sandpowder in a high-temperature atmosphere. Known examples of such a methodinclude what are called the oxygen-hydrogen Verneuil's method, the arcVerneuil's method, the plasma-Verneuil's method and the like, which aredifferent from each other in types of the heat source which forms thehigh-temperature atmosphere. Attempts have been made so as to applythese melting methods to the production of the silica glass crucible andmake substantially eliminate bubbles in the silica glass crucible. Forexample, in publication of examined patent application No. Hei 4-22861,a method of producing a silica glass crucible is proposed in which atransparent layer is formed in the inner surface portion of the crucible(by using the arc Verneuil's method, the inner surface portion is madeto come into direct contact with the liquid of molten silicon during thepulling process).

In publication of unexamined application No. Hei 8-268727, a method ofproducing a quartz crucible is disclosed which method includes the stepsof: centrifuging silica sand put in a melting pot such that the silicasand has the bowl-like shape; heating the silica sand of bowl-likeshape; introducing a rapidly-diffusing gas into the silica sand ofbowl-like shape from the outer surface thereof so as to purge remaininggases contained in voids which are formed between every particles ofsilica sand. In addition, in this method of producing a quartz crucible,vacuum is applied to the bottom of the melting pot of the silica sandsuch that a flow of the rapidly-diffusing gas is generated, in order topurge the remaining gases from the voids of the silica sand.

In the case of the method of producing a silica glass crucible disclosedin the former of the aforementioned two references, a problem, that themethod may not be able to adapt to the current pulling process to asufficient degree because the heat load is increased or the timerequired for the pulling process is prolonged due to use of a largerinner diameter of silicon single crystal or introduction of the“multi-pulling” process, may arise. Accordingly, there has been a demandfor a silica glass crucible in which bubbles present in the innersurface layer are less likely to explode even when the heat load isrelatively large or the time required for the pulling process isrelatively long.

According to the method of producing a quartz crucible disclosed in thelatter of the aforementioned references, the growth of bubbles in thesilica glass crucible during the high-temperature heating process in,for example, manufacturing semiconductor single crystals can beprevented because the gases remaining in the voids of the silica sandare replaced with the rapidly-diffusing gas. However, as the remaininggases (nitrogen gas or oxygen gas) are still not sufficiently replacedwith the rapidly-diffusing gas in this production method, bubblespresent in the opaque layer tend to increase volume thereof when thecrucible is used (i.e., during the pulling process), therebydeteriorating the heat conductivity of the quartz crucible and thusraising the temperature of the quartz crucible. As a result, the bubblesin the transparent layer are also likely to explode.

In general, nitrogen gas and the like trapped in the voids of the silicasand has a larger density than hydrogen gas which is therapidly-diffusing gas. Accordingly, due to the difference between thedensity of nitrogen gas and that of hydrogen gas, hydrogen gas simplypasses through the voids and thus it takes a long time to completereplacing of nitrogen gas with hydrogen gas. As a result, when therapidly-diffusing gas is blown into the silica sand for only a shortperiod, the replacement cannot be performed sufficiently and aconsiderable amount of nitrogen gas and the like are likely to remain inthe voids. These remaining gases become the bubbles present in the innersurface layer of the silica glass crucible. In short, according to thismethod, the number of the bubbles in the crucible remains substantiallythe same as in the conventional method and cannot be reduced.

In addition, the increase in the amount of inputted heat may alsoinfluence the pulling process of semiconductor single crystal. Duringthe pulling process, the silica glass crucible is supported at the outerperiphery thereof by a holding member made of graphite, and the holdingmember is heated by a heater. That is, the semiconductor material in thesilica glass crucible is heated by the heater by way of the holdingmember.

The more transparent the silica glass crucible is, the more effectivelythe heat from the heater is transferred to the semiconductor materials.However, as a plurality of heater elements are provided with a spacebetween each other, if the silica glass crucible is completelytransparent, heat from the heater elements tends to be directlytransferred to the semiconductor materials in the silica glass crucibleand the semiconductor materials located between the heater elements maynot be able to receive heat at a sufficient level. Therefore, in orderto make the heat be evenly transferred, an opaque layer containing anappropriate number of bubbles is formed in the vicinity of the outerperiphery of the silica glass crucible, such that heat rays emitted fromthe heater are dissipated in the multiple directions and thedistribution thereof is made even when the heat rays pass through thesilica glass crucible.

When the number of the bubbles present in the opaque layer or the sizeof such bubbles is too large, it becomes difficult for the heat raysemitted from the heater to reach the transparent layer side and theliquid of molten semiconductor in the crucible may not be able toreceive heat efficiently. When the bubbles increase volume thereof byheating, in particular, heat tends to remain inside the silica glasscrucible due to the decreased heat conductivity and the temperature ofthe silica glass crucible itself may rise up to an extremely high level,in spite that the liquid of molten semiconductor cannot receivesufficient heat.

When the temperature of the silica glass crucible has risen to anextremely high level, devitrification occurs in the vicinity of 1550° C.Further, in case the heat from the heater is not sufficientlytransferred to the liquid of molten semiconductor material in the silicaglass crucible, the temperature of the liquid of molten semiconductormaterial may be locally dropped, there by causing icing (localsolidification).

Therefore, a silica glass crucible, in which the inputted heateffectively acts on the semiconductor materials provided therein andthus the time required for the pulling process can be shortened, anyunusual increase in temperature or icing due to the reflection of theheat from the heater toward the heater side is prevented, and the heatfrom the heater can be evenly transferred to the semiconductor materialsand the melt liquid thereof contained in the silica glass crucible, hasbeen demanded.

Further, according to the method of the publication of unexaminedapplication No. Hei 8-268727, as the opening for applying vacuum isprovided only at the bottom portion of the mold, it is difficult to makethe vacuum degree of inside of the mold higher in a short period oftime.

As described above, it is desirable that the number, the size of bubblespresent in the inner surface layer of a silica glass crucible, and thevolume increasing (expansion) rate of the bubbles (ratio of bubblediameter after/before the pulling process) is reduced, in order toimprove the grow th efficiency of single crystals. Further, it isdesirable that the bubbles present in the silica glass crucible are lesslikely to increase volume thereof, so that a stable heat conductivitycan be obtained for the outer layer as well.

FIG. 2 shows the results of a research in which the relationship betweenthe average diameter of bubbles and yield of the single crystal in theinner surface layer (1 mm) at the corner portions (i.e., the boundaryportions of the bottom portions and the side wall portions) of a silicaglass crucible was studied. The corner portion is studied because thecorner portion, in particular, experiences a relatively large loadduring the single crystal pulling process and thus the bubbles presentin the corner portion are closely related to the yield of the singlecrystal.

The silica glass crucible sample used for the research has an innerdiameter of 22 inches. The yield was obtained in a condition in whichsilicon polycrystal of 100 kg was heat melted with maintaining thetemperature of the liquid surface at about 1430° C., seedbars of siliconsingle crystals were immersed in the melting surface, and silicon singlecrystal of 8 inches were pulled up. Test pieces were collected from theaforementioned silica glass crucible and the diameter of the bubbles atthe corner portions and the yield were investigated. In the results, asshown in FIG. 2, the yield deteriorated when the average diameter of thebubbles was 200 mm or larger.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a method of producing asilica glass crucible, in which: the number, the size, and the expansionratio of the bubbles present in a transparent layer are made small suchthat explosion of the bubbles is prevented and the yield of thesemiconductor single crystal is increased; and the number, the size, andthe expansion ratio of the bubbles present in an opaque layer are setappropriate such that the increase in temperature of the silica glasscrucible is suppressed and the heat efficiency during the pullingprocess of semiconductor single crystal is enhanced.

In the first aspect of the present invention, a method of producing asilica glass crucible, in which silica powder is melted by arc dischargebetween graphite electrodes and a silica glass crucible is formed in arotating mold, comprising the steps of supplying at least one type ofgas selected from the group consisting of hydrogen, oxygen, water vapor,helium, neon gases to the mold; and passing the silica powder throughatmosphere of the at least one type of gas supplied at the previous gassupplying step and then supplying the silica powder to inner surface ofthe mold. Particularly, in the second aspect of the present invention,hydrogen and oxygen gas are supplied to the mold at previous gassupplying step. In the third aspect of the present invention, the silicapowder is dispersed inside the mold, such that the silica powder issoftened in atmosphere of the arc discharge prior to the silica powderreaching the inner surface of the mold. In the forth aspect of thepresent invention, the gas and the silica powder are supplied through adouble-wall cylinder, particularly, the silica powder is suppliedthrough an inner cylinder and the gas is supplied through an outercylinder. In the fifth aspect of the present invention, the silicapowder is supplied with oxygen gas through as inner cylinder, andhydrogen gas supplied through an outer cylinder. In the sixth aspect ofthe present invention, a distal end of the inner cylinder is positionedso as to be retracted with respect to a distal end of the outercylinder. Furthermore, in the seventh aspect of the present invention,of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.

According to these aspects of the present invention, the impurities suchas alkaline-earth metal and heavy metals contained in the suppliedsilica powder are replaced with gases such as hydrogen or combusted in ahigh-temperature atmosphere, and thus the purity of the silica powder isenhanced. In particular, as the atmosphere in the mold reaches ahigh-temperature atmosphere, in which the silica powder can melt in ashort period of time, due to the supplied oxygen gas and hydrogen gas,the degassing process can be easily conducted, thereby significantlysuppressing entry of the bubbles into the product crucible. In addition,as the fine powder of graphite which constitutes the electrode is easilyoxidized at a high temperature and released to the ambient air, entry ofgraphite powder into the product crucible is suppressed. Further, theaforementioned supplied gases are diffused into the product crucible andserves so as to reduce the inner pressure of the bubbles.

In particular, according to the third aspect of the present invention,the silica powder is accumulated, in the softened state, on the meltingsurface of the silica glass which has already been formed. Therefore,the silica powder comes into easy contact with the melting surface andthus the impurities deposited on the melting silica surface are easilyremoved.

According to the fourth and the fifth aspects of the present invention,the silica powder is supplied by blowing it with gases. Therefore, thedirection of supplying the substances can be controlled by adjusting thedirection of the double-wall cylinder. According to the sixth aspect ofthe present invention, the silica powder is surrounded with gases at thedistal end portion of the double-wall cylinder and thus can haveexcellent contact with the gases. According to the seventh aspect of thepresent invention, the temperature of the melting layer of the quartz isprevented from dropping, the melting layer continues to react with theatmospheric gases and thus removal of the impurities which could causegeneration of the bubbles is further accelerated.

The present invention provide following aspects in making an opaquelayer. As an eighth aspect, a method of producing a silica glasscrucible, in which silica powder is melted by arc discharge and a silicaglass crucible is formed in a rotating mold, comprising the steps offorming an accumulated layer of the silica powder on inner surface ofthe mold; supplying helium and/or hydrogen (helium represents themhereinafter) gas to the accumulated layer at predetermined positionslocated in sidewalls and a bottom portion of the mold; starting arcdischarge, after supplying helium gas to the accumulated layer for apredetermined time; stopping supply of helium gas and degassing theaccumulated layer, when a thin film-like melting layer has been formedon the surface of the accumulated layer; and starting again supply ofhelium gas when the accumulated layer has reached a predetermined vacuumstate.

According to the eighth aspect of the present invention, helium gas issupplied into the voids of the accumulated layer in the vacuum state andthe atmosphere in the voids is replaced with helium.

In the ninth aspect of the present invention, putting a cover on theupper opening portion of the mold when the accumulated layer has beenformed and degassing the inside of the mold; supplying helium gas to theinside of the mold when the inside of the mold has reached apredetermined vacuum state; and removing the cover and starting arcdischarge, after a pressure inside the mold has risen to a predeterminedvalue, wherein, after removing the cover, supply of helium gas iscontinued for a predetermined time.

According to the ninth aspect of the present invention, as the mold issealed by a cover, a still higher degree of vacuum can be obtained.Therefore, helium gas which is supplied thereafter is fully spread intothe voids of the accumulated layer and thus the replacement by heliumgas is sufficiently effected.

In the tenth aspect of the present invention, supplying helium gas tothe accumulated layer through predetermined positions located in asidewall and a bottom portion of the mold; starting arc discharge, aftersupplying helium gas to the accumulated layer for a predetermined time;continuing supply of helium gas and degassing the accumulated layerthrough upper portions of the sidewall of the mold, when a thinfilm-like melting layer has been formed on the surface of theaccumulated layer.

Due to this aspect, there arises a flow of helium gas which passesthrough the accumulated layer from the lower side to the upper side,thereby effecting the replacement action allover the accumulated layer.

In the eleventh aspect of the present invention, the positions throughwhich helium gas is supplied when the thin film-like melting layer hasbeen formed is switched to the upper portions of the sidewall of themold, and the accumulated layer is degassed through the positions atwhich helium gas was supplied prior to the arc discharge.

Due to this aspect, there arises a flow of helium gas which passesthrough the accumulated layer from the upper side to the lower side,thereby effecting the replacement action allover the accumulated layer.

In the twelfth aspect of the present invention, putting a cover on theupper opening portion of the mold when the accumulated layer has beenformed and degassing the inside of the mold; supplying helium gas to theinside of the mold through predetermined positions located in sidewallsand a bottom portion of the mold, when the inside of the mold hasreached a predetermined vacuum state; and removing the cover andstarting arc discharge, after a pressure inside the mold has risen to apredetermined value, wherein, supplying of helium gas is continued andthe accumulated layer is degassed at upper portions of the sidewall ofthe mold, when a thin film-like melting layer has been formed on thesurface of the accumulated layer.

In the thirteenth aspect of the present invention, the positions throughwhich helium gas is supplied when the thin film-like melting layer hasbeen formed is switched to the upper portions of the sidewall of themold, and the accumulated layer is degassed through the positions atwhich helium gas was supplied prior to the arc discharge.

According to the eighth through eleventh aspects of the presentinvention, as the voide in the accumulated layer is replaced by heliumgas, bubbles mixed in the surface layer of the silica crucible areprevented from expansion at higher temperature atmosphere (1450˜1700°C.) during the semi-conductor single crystal pulling process. Therefore,heat conductivity of the silica glass crucible is not go down, then thesilica glass crucible is prevented from unusual rise of temperature.

Furthermore, in the fourteenth aspect of the present invention, furthercomprising the steps of: supplying at least one type of gas selectedfrom the group consisting of hydrogen, oxygen, water vapor, helium, neongases to the mold; and passing the silica powder through atmosphere ofthe at least one type of gas supplied at the gas supplying step and thensupplying the silica powder to inner surface of the mold.

In the fifteenth aspect of the present invention silica powder isdispersed inside the mold, such that the silica powder is softened inatmosphere of the arc discharge prior to the silica powder reaching theinner surface of the mold.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of main portions of a device for producing asilica glass crucible according to a first embodiment of the presentinvention.

FIG. 2 is a table which shows a relationship between the diameter of thebubbles and the yield of the crystal.

FIG. 3 is a table in which the bubbles of the silica glass cruciblebefore use is compared with the bubbles of the silica glass crucibleafter use.

FIG. 4 is a graph which shows a relationship between the expansion ratioof the bubbles and the yield of the single crystal yield.

FIG. 5 is a sectional view of main portions of a device for producing asilica glass crucible according to a second embodiment of the presentinvention.

FIG. 6 is a sectional view of main portions of a device for producing asilica glass crucible according to a third embodiment of the presentinvention.

FIG. 7 is a table in which the bubbles present in an opaque layer of thesilica glass crucible before use is compared with the bubbles present inthe opaque layer of the silica glass crucible after use.

FIG. 8 is a sectional view of main portions of a device for producing asilica glass crucible according to a fourth embodiment of the presentinvention.

FIG. 9 is a sectional view of main portions of a device for producing asilica glass crucible according to a fifth embodiment of the presentinvention.

FIG. 10 is a view in which the bubbles present in an opaque layer of thesilica glass crucible of the fourth and the fifth embodiments before useis compared with the bubbles present in the opaque layer of the samesilica glass crucible after use.

BEST MODE FOR EMBODYING THE INVENTION

The present invention will be described hereinafter according to thefollowing embodiments. In a first embodiment, a means for reducing thenumber, the size and the expansion ratio of the bubbles present in atransparent layer formed in the inner surface of a silica glasscrucible, is provided.

First, the number and the size of the bubbles are reduced by eliminatingnitrogen gas, alkaline-earth metal and the like remaining between everyparticles of silica powder as the raw material, because such nitrogengas, alkaline-earth metal and the like are the source of the airbubbles. More specifically, by introducing at least one of the gasesselected from the group consisting of hydrogen, oxygen, water vapor,helium, neon gases and the like (when more than two types of gases areselected, the gases are introduced in the mixed state), a silica glasscrucible having a relatively small number of the bubbles can beobtained. Such a silica glass crucible is obtained because thetemperature inside a mold becomes very high by introducing theaforementioned gas(es) at the time of arc discharge. In a case of theconventional arc Verneuills method in which the aforementioned gas(es)is not introduced, as the temperature inside the mold increases, thesurface of the raw material is sintered in the vicinity of 1650° C. andthen begins to melt in the vicinity of 1750° C. On the other hand, inthe present embodiment, by shortening the time required beforecompleting the sintering process and bringing the material to the meltedstate in a relatively short time of period, the degassing process isaccelerated and thus the number of the bubbles mixed into the cruciblecan be reduced. It should be noted that, in particular, when oxygen gasand hydrogen gas are introduced during the arc discharge, thetemperature inside the mold reaches a desired high temperature rapidly.The degree of sintering of the raw material can be set at a relativelylow level in such a high-temperature atmosphere.

Second, in the present embodiment, the expansion ratio of the bubbles isreduced by reducing the inner pressure of the bubbles. The expansionratio of the bubbles can be reduced by introducing hydrogen gas andoxygen gas during the arc discharge, in a manner similar to the methoddescribed above. The aforementioned gas(es) (hydrogen gas, oxygen gasand the like) which is introduced to the raw material is diffused intothe silica glass crucible and the bubbles present in the silica glasscrucible, and is easily dissolved into the silica glass. Hydrogen,helium and neon are diffused quickly, in particular, and are easilydissolved into the silica glass. The atoms of such gases whoseatom-radius is relatively small can freely travel in the silica matrixof the silica glass. As a result, such gases are easily released out ofthe silica glass under a low-pressure and high-temperature condition,thereby lowering the inner pressure of the bubbles and suppressing theincrease in volume thereof.

Third, in the present invention, the number, and the expansion ratio ofthe bubbles are reduced by eliminating graphite which comes from thegraphite electrodes. A silica glass crucible is produced by melting andsolidifying the raw materials in a mold. The heat needed for melting isobtained from the arc discharge by the graphite electrodes. Most of thegraphite (debris) derived from the graphite electrodes is combined withoxygen in the mold and combusted. However, some of the graphite debrisremains in the produced silica glass crucible, forms the bubbles andserves to increase the expansion ratio of the bubbles. This phenomenonoccurs because the remaining graphite reacts with oxygen which istrapped together in the bubble, generates carbon monoxide gas andincreases the volume of bubbles under the low-pressure andhigh-temperature condition (i.e., the condition during the pullingprocess).

Therefore, in order to prevent graphite from entering the inner surfaceof the silica glass crucible, the gases such as hydrogen, oxygen, watervapor, helium, and neon are introduced into the material at the time ofthe arc discharge. Graphite debris is combined with the introducedoxygen and combusted as described above. In addition, as the other gasesare also introduced as described above, the temperature inside the moldbecomes extremely high and the graphite debris is more easily combinedwith oxygen in the air and more easily combusted. Hydrogen, oxygen,water vapor particularly accelerate the oxidation of graphite.

Next, a specific method of producing a silica glass crucible and theresults of measurement of the expansion ratio of the bubbles present ina transparent layer of a produced silica glass crucible will bedescribed. In the first embodiment, hydrogen gas is introduced into themold. FIG. 1 is a sectional view of main portions of a device forproducing a silica glass crucible. In FIG. 1, a mold 1 has a double-wallstructure made of metal (preferably, stainless steel), and is adapted tobe water cooled by cooling water which flows inside a tube 1B providedin an inner wall 1A. The inner diameter of the mold 1 is 570 mm and themold 1 is rotated around a shaft 2 by a rotating device (not shown). Apair of graphite electrodes 4, 5 are retained on a heat-blocking plate(cover) 3 provided at the center-upper portion of the mold 1.

A gas nozzle 6 and an introducing tube 7 for introducing silica powderas the raw material form a double-wall cylinder, and this double-wallcylinder is provided adjacent to the graphite electrode 4. Theintroducing pipe 7 forms the inner wall (cylinder) of the double-wallcylinder and the gas nozzle 6 forms the outer wall (cylinder) of thedouble-wall cylinder. However, the gas nozzle 6 may form the inner wall(cylinder) of the double-wall cylinder and the introducing pipe 7 mayform the outer wall (cylinder) of the double-wall cylinder. When thedouble-wall cylinder is structured so as to introduce silica powderthrough the inner cylinder and introduce gases through the outercylinder, it is preferable that the inner and outer cylinders arestructured such that the distal end of the inner cylinder is retractedwith respect to the distal end of the outer cylinder (i.e., such thatthe distal end of the inner cylinder is completely covered with theouter cylinder) as shown in FIG. 1. By structuring the double-wallcylinder in such a manner, silica powder is fully surrounded with gasesand reliably comes in excellent contact with the gases.

In the present embodiment, silica powder is used as the raw material.However, what is represented by the term “silica powder” is not limitedto quartz but also includes powder materials known as the raw materialsfor a silica glass crucible, such as crystals, siliceous sand and thelike which contains silicon dioxide (silica). The inlet 6A of the gasnozzle 6 is connected to pipelines for hydrogen gas or a gas cylinder(not shown) and the upper end of the introducing tube 7 is connected toa hopper 8 in which silica powder is provided. A gas introducing tube 9may additionally be provided between the graphite electrode 4 and thegraphite electrode 5.

Process of producing a crucible by the producing device structured asdescribed above will be described hereinafter. First, the mold 1 isrotated and silica powder (whose particle degree is 60# (meshes perinch)—150#, for example) is provided to the inner peripheral surface ofthe mold 1 by way of the introducing tube 7. As the mold is beingrotated, the provided silica powder is attached to the inner(peripheral) surface of the mold 1 due to the centrifuging force andaccumulated thereon. The silica powder may be provided to the mold 1 byeither the introducing tube 7 or any other suitable device. When apredetermined amount of silica powder has been accumulated, anelectricity is applied between the graphite electrodes 4, 5 such thatthe arc discharge is effected. The silica powder which has been attachedto the mold 1 is melted due to the arc heat and an opaque layer L1(refer to FIG. 1), which constitutes the outer peripheral surface of thesilica glass crucible, is formed.

After the formation of the opaque layer L1, a transparent layer L2(refer to FIG. 1) is subsequently formed. 5 to 10 minutes after thestart of melting of the accumulated silica powder, the silica powder andhydrogen gas are supplied to the mold 1 by passing through theintroducing tube 7 and the gas nozzle 6. Here, it is preferable that thesilica powder is provided at the rate of 80 to 160 g/minute and hydrogengas is provided at the rate of 60 to 100 liter/minute.

The temperature of the atmosphere in the vicinity of the arc of thegraphite electrodes 4, 5 is 2000° C. or higher (5000° C. or higher inthe arc) and the silica powder dispersed in the atmosphere is softened.The softened silica powder is directly transferred onto the opaque layerL1 or is raised up to the opaque layer L1 due to the centrifuging forceonce it is dropped to the bottom of the mold, and is accumulated on theopaque layer L1, thereby forming the transparent layer L2.

The silica powder, which has reached the softened state after beingapplied together with hydrogen gas in the vicinity of the arc, increasesits purity because the impurities attached to the surface of the silicapowder such as air, moisture, alkaline-earth metals and heavy metals arereplaced with hydrogen gas or combusted in the high-temperatureatmosphere. As a result, the amount of the gas trapped in thetransparent layer L2 (the remaining gas) which is potentially the sourceof the bubbles can be reduced. In addition, if some amount of gas istrapped in the transparent layer, since such a remaining gas is reliablyreplaced with hydrogen gas which is highly diffusive (hydrogen gas cantravel freely in the silica matrix of the silica glass as describedabove), the remaining gas is easily released out of the silica glassunder the condition of low-pressure and high-temperature. Accordingly,explosion due to volume increase of bubbles is unlikely to occur.

In order to reduce the amount of the remaining gas trapped in thetransparent layer L2 as much as possible, it is preferable that thetemperature of the quartz surface layer in the melted state ismaintained at a high temperature (2000° C. or higher). The higher thetemperature is, the more remaining gas is released from the meltedsurface layer to the atmosphere. It is preferable that the silica powderis intermittently dispersed, because, if silica powder is continuallyprovided, the temperature of the surface layer in the melted state maydrop. Accordingly, dispersion of the silica powder and introduction ofhydrogen gas are carried out for 10 minutes, and then dispersion of thesilica powder is stopped for 20 minutes while continuing the meltingoperation during the period, for example. By repeating this operation,the transparent layer L2 having a predetermined thickness can be formed.It should be noted that introduction of hydrogen may be continued whilethe dispersion of the silica powder is stopped, so that hydrogen gascontinues to act on the remaining gas of the melted surface layer.

With respect to a silica glass crucible produced as described above, thenumber, the size, and the volume increasing rate of the bubbles wereinvestigated. FIG. 3 is a table which shows the results of a research inwhich the number and the size of the bubbles after use of the crucibleis compared with the number and the size of the bubbles before use, withrespect to a silica glass crucible produced by the production method ofthe present embodiment. The bubbles before use of the crucible wereobserved, by setting a microscope (with a magnifying power of 20) at aninner corner portion of the silica glass crucible and observing the areadepthwise between the inner peripheral surface and the 0.5 to 1.5 mmdepth point at each corner portion, with respect to 20 sites (samples).

The observation of the bubbles after use of the crucible was carried outwith respect to the samples, which were obtained by cutting the cornerportions of the silica glass crucible after use off, etching the cutsection with 15% acid ammonium fluoride for 30 minutes at the roomtemperature and washed by super pure water shower. The observation wascarried out, in a similar manner to the before-use case, at the areadepthwise between the inner peripheral surface and the 0.5 to 1.5 mmdepth point at a corner portion of the silica glass crucible, withrespect to 20 sites (samples). Here, the sample of “after use”represents the silica glass crucible which has been used for a pullingprocess in which 100 kg of silicon was pulled after 50 hours accordingto the CZ method.

As shown in FIG. 3, the maximum size (diameter) of the bubbles beforeuse was 51 μm, the minimum size (diameter) thereof was 3 μm, the averagewas 13 μm. The maximum size (diameter) of the air bubbles after use was90 μm, the minimum size (diameter) thereof was 9 μm, the average was 34μm. The number of the air bubbles present in the silica glass cruciblewas 0.14/mm3 before use and 0.27/mm3 after use. As described above, theaverage size of the bubbles after use was 34 μm. The yield of the singlecrystal was 100%, matching with the results described with respect toFIG. 2.

Next, the results of a research which investigated the relationshipbetween the volume increasing rate of the bubbles and the yield will bedescribed hereinafter. According to the aforementioned productionmethod, but with varying the introduced amount of hydrogen gas between 0to 60 liter/minute, 13 types of silica glass crucibles of 22 inches(diameter) were produced. With respect to these silica glass crucibles,the volume increasing rate of the bubbles (bubble diameter afteruse/bubble diameter before use) and the yield of the single crystal wereinvestigated and the relationship between the two rates were studied.FIG. 4 is a graph which shows the relationship between the volumeincreasing rate of the bubbles and the yield of the single crystal. Whenthe yield was investigated, the conditions with respect to theuse/observation of the silica glass crucibles were the same as thosedescribed with respect to FIG. 3.

As shown in FIG. 4, the volume increasing rate of the bubbles of the 13types of silica glass crucibles distributes in a range of 1.5 to 3.5. Asunderstood from FIG. 4, the samples in which the volume increasing rateof the bubbles is less than “2.5%” show an excellent result of the yieldof the single crystal (100%).

As described above, according to the present embodiment, contaminantsresulted from factors outside the operation environment (typically,trapping of the ambient air) and impurities contained in the rawmaterial powder are intensively eliminated. As a result, the amount ofinactive gases trapped in the transparent layer of the silica glasscrucible can be reduced to an extremely or ultimately low level.

Next, a second embodiment of the present invention will be described. Inthe aforementioned first embodiment, the crucible is produced byintroducing hydrogen gas. In the second embodiment, oxygen gas andhydrogen gas are introduced. FIG. 5 is a sectional view of main portionsof a device for producing a silica glass crucible according to thesecond embodiment. The same reference numbers as in FIG. 1 aredesignated to the portions of FIG. 5 which are the same as or equivalentto FIG. 1. In FIG. 5, a double-wall cylinder 70 is formed by anintroducing tube 70A as the inner cylinder and an gas nozzle 6 as theouter cylinder. The introducing tube 70A as the inner cylinder isstructured so as to be able to supply the raw material powder by way ofa hopper 8. The introducing tube 70A also has a branched tube or aninlet 70B at midway thereof so as to be able to introduce oxygen gas. Onthe other hand, the gas nozzle 6 as the outer cylinder of thedouble-wall cylinder 70 has an inlet 6A for introducing hydrogen gas, ina manner similar to the first embodiment.

In the aforementioned structure, as the silica powder and oxygen gas areinjected together from one common tube (i.e., the introducing tube 70A),the silica powder can be supplied to the mold from any desired directionby adjusting the direction of the introducing tube 70A. However, thepresent invention is not limited to this structure, and the silicapowder and oxygen gas may be separately supplied to the mold 1 by usingdifferent nozzles.

In operation, an opaque layer L1 is formed at first in a manner similarto the first embodiment. After forming the opaque layer L1, atransparent layer L2 (refer to FIG. 1) is subsequently formed. 5 to 10minutes after the start of melting of the accumulated silica powder,silica powder, oxygen gas and hydrogen gas are supplied to the mold 1through the double-wall cylinder 70. Here, similarly to the firstembodiment, it is preferable that the silica powder is supplied at therate of 80 to 160 g/minute, for example, and oxygen gas and hydrogen gasare supplied as a whole at the rate of 60 to 100 liter/minute. Inaddition, it is preferable that the ratio of the supply amount of oxygengas with respect to the supply amount of hydrogen gas is 1:6. When thesupply amount of oxygen gas is too large, the amount of the bubbles mayrather increase. When the supply amount of hydrogen gas is to beincreased at the maximum level, the aforementioned ratio between oxygenand hydrogen can be 1:10.

The silica powder dispersed in the atmosphere in the vicinity of the arcof the graphite electrodes 4, 5 is softened. The softened silica powderis directly transferred onto the opaque layer L1 or is raised up to theopaque layer L1 due to the centrifuging force once it is dropped to thebottom of the mold, and is accumulated on the opaque layer L1, therebyforming the transparent layer L2. In particular, the silica powder,which has passed through the atmosphere in which the temperature hasrisen high as a result of introduction of oxygen and hydrogen, softensvery easily.

The transparent layer L2 formed by the silica powder which has reachedthe softened state as a result of being introduced in the vicinity ofthe arc with oxygen gas and hydrogen gas, has a relatively small amountof remaining gas which becomes the source of the bubbles. Even if asmall amount of gas remains in the transparent layer L2, as theenvironment is hydrogen rich, the remaining gas is easily purged out ofthe silica glass under a low-pressure and high-temperature condition ina manner similar to the case in which only hydrogen gas is introduced.

When a silica glass crucible is produced in a condition in whichhydrogen gas flow ratio is high, the inside of the crucible is in thehydrogen rich state, as described above. As hydrogen gas is present soamply, the hydrogen rich state inside the silica glass crucible canreliably be maintained, although some hydrogen gas reacts with oxygengas which enters the silica glass crucible from the air due to theconvection generated during the pulling process of semiconductor singlecrystals, to form a vapor. The vapor generated during the pullingprocess is released from the upper portion of the silica glass crucibledue to the high-temperature convection of the arc discharge, withoutcoming into contact with the inner surface of the silica glass crucible.

The results of the comparison of the number and the size of the bubblespresent in the silica glass crucible of the second embodiment after use,with those of the silica glass crucible before use, are similar to theresults obtained in the first embodiment. In addition, the results ofthe volume increasing rate of the bubbles in the same silica glasscrucible were similar to the results obtained in the first embodiment.

In the second embodiment, oxygen gas is introduced together with the rawmaterial powder through the inner cylinder of the double-wall cylinderand hydrogen gas is introduced through the outer cylinder. However, thisstructure may be constructed in a reversed manner. That is, hydrogen gasmay be introduced together with the raw material powder through theinner cylinder and oxygen gas may be introduced through the outercylinder. Further, the present embodiment is not limited to thestructure in which oxygen gas and hydrogen gas are introducedseparately. Both gases may be mixed in advance, so that the mixed gas isintroduced.

Next, a production method for stabilizing the heat conductivity in theouter surface layer or the opaque layer of the silica glass cruciblewill be described. In order to make the heat conductivity of the opaquelayer stable, helium gas is introduced from the pot or the mold formelting the silica powder and the gases in the voids of the silicapowder accumulated in the inner wall surface of the mold is replace withhelium gas. The details thereof will be described hereinafter withreference to the drawings.

FIG. 6 is a sectional view of main portions of a production device whichis used in the method for producing a silica glass crucible according toa third embodiment. The same reference numbers as in FIG. 1 aredesignated to the portions of FIG. 6 which are the same as or equivalentto FIG. 1. The end portion of a cooling water pipe 1B fixed to an innerwall 1A of the hollow portion of the mold 1, is extended downward themold 1 and connected to a circulation device (not shown). Bores 10 forintroducing helium as a replacing gas are formed at the bottom portionof the inner wall 1A of the mold 1. It should be noted that the bores 10for introducing helium gas may be formed not only at the bottom portionbut also at any positions between the bottom portion and the side wallof the mold 1 (refer to FIG. 9 which will be described later). Thehollow portion of the double-wall mold 1 is filled with helium gas,which is supplied from the bottom portion of the mold 1. The helium gasfilled in the hollow portion upwardly passes through the inner wall 1Aby way of the holes 10. On the other hand, holes 11 for discharginggases are formed in the inner wall in the vicinity of the upper end ofthe mold 1 (that is, at the portion in the vicinity of the upper openingof the mold 1). Each hole 11 is connected with a joint 12, and a tube 13which extends downward of the mold 1 and communicates with a vacuum pump(not shown) is connected to the joint 12. The tube 13 may be structuredsuch that the tube 13 passes through the hollow portion of the mold 1and is extended downwardly.

The inner diameter of the mold 1 is 570 mm, for example, and is rotatedby a rotating device (not shown) as shown in the arrow 14. Accumulatedlayer 15 of the raw material is formed on the wall surface of the mold1, prior to the generation of the arc between the electrodes 4, 5. Theinlet 6A of the gas nozzle 6 is connected to hydrogen gas pipelines or agas cylinder (not shown). The upper end of the introduction tube 7 isconnected to a raw material hopper accommodating silica powder, in amanner similar to the production device of FIG. 1.

The process steps required when a silica glass crucible is produced byusing the aforementioned production device will be described. First, theaccumulated layer 15 of the silica powder is formed. The preferableparticle degree and amount to be supplied of the silica powder aresimilar to those of the first embodiment. When the accumulated layer 15having a predetermined thickness has been formed, the melting process ofthe accumulated layer 15 is started by effecting the arc dischargebetween the graphite electrodes 4, 5.

Prior to the arc discharge, introduction of helium gas to theaccumulated layer 15 is started by way of the bores 10 of the mold 1. Itis preferable that the helium gas is supplied at the rate of 30liter/minute or so. After supplying helium gas for approximately 5minutes, the arc discharge is conducted. A thin melting layer is formedon the surface of the accumulated layer 15 in 1 to 2 minutes afterstarting the arc discharge. The introduction of helium gas is stoppedwhen this thin film-like melting layer has been formed, and the portionof the accumulated layer 15 which has not been melted is degassed by wayof the holes 11. When the degree of vacuum inside the mold 1 has reacheda predetermined level (30 torr, for example) due to this degassingprocess, introduction of helium gas is started again. It is alsoacceptable that the supply of helium gas is not completely stopped whenthe thin film-like melting layer is formed and the supply of helium gasis continued at a low rate (less than 10 liter/minute).

As helium gas is introduced when the degree of vacuum has reached a highlevel in a third embodiment, helium gas is effectively absorbed to theportion of the accumulated layer 15 which has not been melted and thusthe air in the voids (space) of the accumulated layer 15 is effectivelyreplaced with the helium gas. Since the air in the voids of theaccumulated layer 15 is replaced with helium gas, the bubbles present inthe opaque layer which is formed as a result of the melting of theaccumulated layer 15 contain very little amount of nitrogen gas andoxygen gas, thereby showing a relatively small volume increasing rate.It should be noted that, the introduction of helium gas which is startedagain after the degassing process may be stopped after a predeterminedtime or may be continued at a low supply rate (less than 10liter/minute).

After an opaque layer having thickness of 10 mm or so has been formed, atransparent layer is formed on the opaque layer. More precisely, atransparent layer is formed on the silica glass crucible inner surfaceside of the opaque layer. The transparent layer is formed, in a mannersimilar to that of the first embodiment, in which hydrogen gas isintroduced through the gas nozzle 6 simultaneous with dispersing thesilica powder to the mold 1 through the introducing tube 7.

FIG. 7 is a table showing the results of a research conducted withrespect to the number, the diameter size, and the volume increasing rateof the bubbles present in the opaque layer. The observation of thebubbles in the crucible before use/after use was carried out in a mannersimilar to that of the transparent layer research. The unit of thenumber of the bubbles is (number)/mm3, and the unit of the diameter sizeof the bubbles is μm. As shown in FIG. 7, the number and the diametersize of the bubbles present in the crucible before use are substantiallythe same in both the conventional silica glass crucible and the improvedsilica glass crucible produced by the production method of the presentthird embodiment. However, with respect to the diameter size of eachbubble, a significant improvement is observed in the improved silicaglass crucible according to the present third embodiment. The number,the diameter size, and the volume increasing rate of the bubbles presentin the transparent layer of the silica glass crucible of the thirdembodiment were similar to those of the first embodiment.

A modified example of the method of forming the opaque layer will bedescribed. FIG. 8 and FIG. 9 are sectional views of devices employed inthe production method according to the present modified example. FIG. 8and FIG. 9 each show only the main portions of the mold 1 and theelectrodes 4, 5, the cooling water pipeline 1B and the like are notshown. The same reference numbers as in FIG. 6 are designated to theportions of FIGS. 8 and 9 which are the same as or equivalent to FIG. 6.In a device of a fourth embodiment shown in FIG. 8, bottom portion holes10 are formed at the bottom portion of the inner wall 1A of the mold 1,and a side bore 16 is formed at the side portion. A vacuum pump 18 andhelium gas supply source 19 are connected to the bottom of the mold 1 byway of a changeover switch means 17. A cover 20 is put on the mold 1until the arc discharge by the electrodes 4, 5 is started.

After the accumulated layer 15 is formed in this manufacturing device,the cover 20 is put on the upper opening portion of the mold 1, and theinside of the mold 1 is degassed through the bottom portion holes 10 andthe side holes 16 by driving the vacuum pump 18. When the inside of themold 1 has reached the predetermined vacuum state or degree (1 to 5torr, for example), the switch means 17 is switched to the side ofhelium gas supply source 19, such that helium gas is supplied to theinside of the mold 1 by way of the bottom portion holes 10 and the sideholes 16. When the pressure inside the mold 1 has risen to thepredetermined level (for example, when the pressure inside the mold 1has reached the ambient atmosphere), the process is shifted to the stepof opening the cover 20 and starting the arc discharge. The introductionof helium gas is continued even after the cover 20 is opened and the arcdischarge is started. It is preferable that the introduction of heliumgas is continued at least until the thin film like-melting layer isformed on the surface of the accumulated layer 15.

On the other hand, in the device of a fifth embodiment shown in FIG. 9,bottom portion holes 10 are formed at the bottom portion of the innerwall 1A of the mold 1 and, in the side portion of the device, an upperportion holes 11 are further formed above the side holes 16 (in additionto the side holes 16). The side holes 16 communicate with the bottomportion holes 10. In the device shown in FIG. 9, after the accumulatedlayer 15 has been formed on the inner surface of the mold 1, helium gasis supplied to the accumulated layer 15 by way of the side holes 16 andthe bottom portion holes 10, and after supplying helium gas for apredetermined time (for 5 minutes, for example), the arc discharge isperformed. When a thin film-like melting layer has been formed on thesurface of the accumulated layer 15 due to the arc discharge, theaccumulated layer 15 is degassed through the upper portion holes 11formed at the upper portion of the side wall of the mold 1, while theintroduction of helium gas is continued. As a result of the supply ofhelium gas and degassing described above, there arises a flow of heliumgas which flows in the accumulated layer 15 from the lower portion tothe upper portion thereof. Gases remaining in the voids of theaccumulated layer 15 is replaced with the helium gas introduced in sucha manner.

The aforementioned flow of helium gas may be directed so as to flow inthe accumulated layer 15 from the upper portion to the lower portionthereof. For example, after supplying helium gas for a predeterminedtime (5 minutes, for example) to the accumulated layer 15 through theside holes 16 formed in the side wall and the bottom portion holes 10formed in the bottom portion of the mold 1, the process is shifted tothe step of starting the arc discharge. Then, when the thin film-likemelting layer is formed on the surface of the accumulated layer 15, thesupply position of helium gas is switched to the upper portion holes 11formed in the side wall of the mold 1, and degassing is carried out withrespect to the accumulated layer 15 at the position where helium gas wassupplied before the arc discharge (i.e., through the bottom portionholes 10 and the side holes 16 of the mold 1). As a result, helium gasbegins to flow downwardly. In order to change the positions at whichsupply/degassing of helium gas is carried out, the helium gas supplysource 19 is connected to two tubes 21, 22 extended from the bottomportion of the mold 1, by way of a changeover switch means 23.

In a case in which a flow of helium gas is generated in the accumulatedlayer 15 by using the device shown in FIG. 9, as is in the modifiedexample shown in FIG. 8, it is acceptable that the cover 20 is put onthe mold 1 so as to form a sealed space, the inside of the mold 1 isvacuumed prior to generation of the arc discharge and the atmospheretherein is replaced with helium gas by introducing helium gas.

A film containing water is present around the silica powder. Therefore,it is preferable that such moisture is removed in advance. Moisture canbe removed by pre-heating the accumulated layer 15 prior to generationof the arc. For example, by (removably) providing a halogen lamp in themold 1 so that the halogen lamp is directed to the accumulated layer 15and raising the temperature inside the mold 1 to about 100° C., theaccumulated layer 15 can be heated during the degassing process. Byremoving moisture around the silica powder in advance, the replacementprocess by helium gas can be carried out in a shorter time.Alternatively, the accumulated layer 15 may be heated by heatersincorporated in the cover 3 or the cover 20.

FIG. 10 is a table showing the results of a research conducted withrespect to the number, the diameter size, and the volume increasing rateof the bubbles present in the opaque layer of the silica glass crucibleproduced by the production methods of the fourth and the fifthembodiments. In FIG. 10, the second sample II is the silica glasscrucible obtained by the fourth embodiment, in which helium gas isintroduced so as to generate the arc after the mold 1 with the cover 20has been vacuumed to 5 torr. The third sample III is the silica glasscrucible obtained by the fifth embodiment, in which a flow of helium gasis generated in the accumulated layer 15 which is vacuumed to 160 torr.The sample silica glass crucible III exhibits a higher volume increasingrate of the bubbles than that of the sample silica glass crucible II.However, the relatively high volume increase rate of the bubbles of thesample silica glass crucible III can be lowered by heating theaccumulated layer 15 by the aforementioned halogen lamp or the like whenthe sample III is produced. The volume increasing rate of the sample IIcan also be further lowered by likewise heating the accumulated layer15.

As described above, according to the present embodiment, contaminantsresulted from factors outside the operation environment (typically,trapping of the ambient air) and impurities contained in the rawmaterial powder are intensively eliminated. As a result, the amount ofinactive gases trapped in the transparent layer of the silica glasscrucible can be reduced to an extremely or ultimately low level.

Although a case in which hydrogen gas is used as the introducing gas hasbeen described above, similar excellent results (improvement in thenumber, the diameter size and the like of the bubbles, and better yieldof single crystals) can be likewise obtained in cases in whichintroducing gases other than hydrogen gas (at least one type of gasselected from the group consisting of oxygen, water vapor, helium andneon) are used under the same conditions.

Further, with respect to the gas supplied from the mold 1 in the thirdto fifth embodiments, the type of the gas is not limited to helium gas,and other types of rapidly-diffusing gas (such as hydrogen) or themixture of helium gas and hydrogen may be used.

Industrial Applicability

As is obvious from the aforementioned description, according to theinvention defined by claims 1 to 8, by introducing gas(es) such asoxygen and hydrogen into the arc, the influence of the high temperaturecan be brought not only to the silica powder being dispersed but also tothe melting layer, thereby allowing effective removal of impuritiespresent in the silica powder. As a result, bubbles derived from theimpurities are reduced, and thus entry of the bubbles into the innersurface layer of the crucible can be effectively prevented. In addition,even if some bubbles enter or are trapped in the inner surface layer, asmost of the trapped bubbles are replaced with the introducing gas(es),the inner pressure of the bubbles is likely to be reduced. That is, asilica glass crucible in which bubbles are not likely to explode evenunder a high-temperature load is provided, and a higher yield can beexpected in the pulling of large-diameter semiconductor single crystals.

Further, according to the invention defined by claims 9 to 16, gasesremaining in the voids of the accumulated layer of the silica powder asthe raw material is replaced with helium gas. Therefore, in case somebubbles are trapped in the opaque layer of the silica glass crucible,the volume increase rate of these bubbles is likely to be small and theheat conductivity can be kept moderate when the silica glass crucible isused at a high temperature. As a result, unusual rise in temperature ofthe silica glass crucible can be avoided and volume increasing/explosionof the bubbles in the transparent layer can be prevented.

What is claimed is:
 1. A method of producing a silica glass crucible, inwhich silica powder is melted by arc discharge between graphiteelectrodes and a silica glass crucible is formed in a rotating mold,comprising the steps of: supplying at least one type of gas selectedfrom the group consisting of hydrogen, oxygen, water vapor, helium, andneon gases to the mold; and passing the silica powder through atmosphereof the at least one type of gas supplied at the previous gas supplyingstep and then supplying the silica powder to an inner surface of themold.
 2. A method of producing a silica glass crucible, in which silicapowder is melted by arc discharge between graphite electrodes and asilica glass crucible is formed in a rotating mold, comprising the stepsof: supplying hydrogen and oxygen gases to the mold; and passing thesilica powder through atmosphere of the hydrogen and oxygen gasessupplied at the previous gas supplying step and then supplying thesilica powder to an inner surface of the mold.
 3. A method of producinga silica glass crucible described in claim 1, wherein the silica powderis dispersed inside the mold, such that the silica powder is softened inatmosphere of the arc discharge prior to the silica powder reaching theinner surface of the mold.
 4. A method of producing a silica glasscrucible described in claim 2, wherein the silica powder is dispersedinside the mold, such that the silica powder is softened in atmosphereof the arc discharge prior to the silica powder reaching the innersurface of the mold.
 5. A method of producing a silica glass crucibledescribed in any of claims 1 to 3, wherein the gas and the silica powderare supplied through a double-wall cylinder which is formed by an outercylinder and an inner cylinder contained in the outer cylinder, one ofthe silica powder and the gas is supplied through the inner cylinder,and the other of the silica powder and the gas is supplied through theouter cylinder.
 6. A method of producing a silica glass crucibledescribed in claim 4, wherein the gas and the silica powder are suppliedthrough a double-wall cylinder which is formed by an outer cylinder andan inner cylinder contained in the outer cylinder, one of the silicapowder and the gas is supplied through the inner cylinder, and the otherof the silica powder and the gas is supplied through the outer cylinder.7. A method of producing a silica glass crucible described in any ofclaims 1 to 3, wherein the gas and the silica powder are suppliedthrough a double-wall cylinder which is formed by an outer cylinder andan inner cylinder contained in the outer cylinder, the silica powder issupplied through the inner cylinder, and the gas is supplied through theouter cylinder whose distal end extends beyond a distal end of the innercylinder.
 8. A method of producing a silica glass crucible described inclaim 4, wherein the gas and the silica powder are supplied through adouble-wall cylinder which is formed by an outer cylinder and an innercylinder contained in the outer cylinder, the silica powder is suppliedthrough the inner cylinder, and the gas is supplied through the outercylinder whose distal end extends beyond a distal end of the innercylinder.
 9. A method of producing a silica glass crucible described inclaim 2 or claim 3, wherein the gas and the silica powder are suppliedthrough a double-wall cylinder which is formed by an outer cylinder andan inner cylinder contained in the outer cylinder, the silica powder andoxygen gas are supplied through the inner cylinder, and the hydrogen gasis supplied through the outer cylinder.
 10. A method of producing asilica glass crucible described in claim 4, wherein the gas and thesilica powder are supplied through a double-wall cylinder which isformed by an outer cylinder and an inner cylinder contained in the outercylinder, the silica powder and oxygen gas are supplied through theinner cylinder, and the hydrogen gas is supplied through the outercylinder.
 11. A method of producing a silica glass crucible described inclaim 9, wherein a distal end of the inner cylinder is positioned so asto be retracted with respect to a distal end of the outer cylinder. 12.A method of producing a silica glass crucible described in claim 10,wherein a distal end of the inner cylinder is positioned so as to beretracted with respect to a distal end of the outer cylinder.
 13. Amethod of producing a silica glass crucible described in claim 1,wherein, of the silica powder and the gas, at least the silica powder isintermittenly supplied, while the arc discharge is continued.
 14. Amethod of producing a silica glass crucible described in claim 2,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 15. Amethod of producing a silica glass crucible described in claim 2,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 16. Amethod of producing a silica glass crucible described in claim 4,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 17. Amethod of producing a silica glass crucible described in claim 5,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 18. Amethod of producing a silica glass crucible described in claim 6,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 19. Amethod of producing a silica glass crucible described in claim 7,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 20. Amethod of producing a silica glass crucible described in claim 8,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 21. Amethod of producing a silica glass crucible described in claim 9,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 22. Amethod of producing a silica glass crucible described in claim 10,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 23. Amethod of producing a silica glass crucible described in claim 11,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 24. Amethod of producing a silica glass crucible described in claim 12,wherein, of the silica powder and the gas, at least the silica powder isintermittently supplied, while the arc discharge is continued.
 25. Amethod of producing a silica glass crucible, in which silica powder ismelted by arc discharge and a silica glass crucible is formed in arotating mold, comprising the steps of: forming an accumulated layer ofthe silica powder on an inner surface of the mold; supplying heliumand/or hydrogen gas to the accumulated layer at predetermined positionslocated in sidewalls and a bottom portion of the mold; starting arcdischarge, after supplying helium and/or hydrogen gas to the accumulatedlayer for a predetermined time; stopping supply of helium and/orhydrogen gas and degassing the accumulated layer, when a thin film-likemelting layer has been formed on the surface of the accumulated layer;and starting again supply of helium and/or hydrogen gas when theaccumulated layer has reached a predetermined vacuum state.
 26. A methodof producing a silica glass crucible, in which silica powder is meltedby arc discharge and a silica glass crucible is formed in a rotatingmold, comprising the steps of: forming an accumulated layer of thesilica powder on an inner surface of the mold; putting a cover on theupper opening portion of the mold when the accumulated layer has beenformed and degassing the inside of the mold; supplying helium and/orhydrogen gas to the inside of the mold when the inside of the mold hasreached a predetermined vacuum state; and removing the cover andstarting arc discharge, after a pressure inside the mold has risen to apredetermined value, wherein, after removing the cover, supply of heliumand/or hydrogen gas is continued for a predetermined time.
 27. A methodof producing a silica glass crucible, in which silica powder is meltedby arc discharge and a silica glass crucible is formed in a rotatingmold, comprising the steps of: forming an accumulated layer of thesilica powder on an inner surface of the mold; supplying helium and/orhydrogen gas to the accumulated layer through predetermined positionslocated in a sidewall and a bottom portion of the mold; starting arcdischarge, after supplying helium and/or hydrogen gas to the accumulatedlayer for a predetermined time; continuing supply of helium and/ordegassing the accumulated layer through upper portions of the sidewallof the mold, when a thin film-like melting layer has been formed on thesurface of the accumulated layer.
 28. A method of producing a silicaglass crucible described in claim 27, wherein the positions throughwhich helium and/or hydrogen gas is supplied when the thin film-likemelting layer has been formed is switched to the upper portions of thesidewall of the mold, and the accumulated layer is degassed through thepositions at which helium and/or hydrogen gas was supplied prior to thearc discharge.
 29. A method of producing a silica glass crucible, inwhich silica powder is melted by arc discharge and a silica glasscrucible is formed in a rotating mold, comprising the steps of: formingan accumulated layer of the silica powder on an inner surface of themold; putting a cover on the upper opening portion of the mold when theaccumulated layer has been formed and degassing the inside of the mold;supplying helium and/or hydrogen gas to the inside of the mold throughpredetermined positions located in sidewalls and a bottom portion of themold, when the inside of the mold has reached a predetermined vacuumstate; and removing the cover and starting arc discharge, after apressure inside the mold has risen to a predetermined value, wherein,supplying of helium and/or hydrogen gas is continued and the accumulatedlayer is degassed at upper portions of the sidewall of the mold, when athin film-like melting layer has been formed on the surface of theaccumulated layer.
 30. A method of producing a silica glass crucibledescribed in claim 29, wherein the positions through which helium and/orhydrogen gas is supplied when the thin film-like melting layer has beenformed is switched to the upper portions of the sidewall of the mold,and the accumulated layer is degassed through the positions at whichhelium and/or hydrogen gas was supplied prior to the arc discharge. 31.A method of producing a silica glass crucible described in claim 25,further comprising the steps of: supplying at least one type of gasselected from the group consisting of hydrogen, oxygen, water vapor,helium, and neon gases to the mold; and passing the silica powderthrough atmosphere of the at least one type of gas supplied at the gassupplying step and then supplying the silica powder to an inner surfaceof the mold.
 32. A method of producing a silica glass crucible describedin claim 26, further comprising the steps of: supplying at least onetype of gas selected from the group consisting of hydrogen, oxygen,water vapor, helium, and neon gases to the mold; and passing the silicapowder through atmosphere of the at least one type of gas supplied atthe gas supplying step and then supplying the silica powder to an innersurface of the mold.
 33. A method of producing a silica glass crucibledescribed in claim 27, further comprising the steps of: supplying atleast one type of gas selected from the group consisting of hydrogen,oxygen, water vapor, helium, and neon gases to the mold; and passing thesilica powder through atmosphere of the at least one type of gassupplied at the gas supplying step and then supplying the silica powderto an inner surface of the mold.
 34. A method of producing a silicaglass crucible described in claim 28, further comprising the steps of:supplying at least one type of gas selected from the group consisting ofhydrogen, oxygen, water vapor, helium and neon gases to the mold; andpassing the silica powder through atmosphere of the at least one type ofgas supplied at the gas supplying step and then supplying the silicapowder to an inner surface of the mold.
 35. A method of producing asilica glass crucible described in claim 29, further comprising thesteps of: supplying at least one type of gas selected from the groupconsisting of hydrogen, oxygen, water vapor, helium, and neon gases tothe mold; and passing the silica powder through atmosphere of the atleast one type of gas supplied at the gas supplying step and thensupplying the silica powder to an inner surface of the mold.
 36. Amethod of producing a silica glass crucible described in claim 30,further comprising the steps of: supplying at least one type of gasselected from the group consisting of hydrogen, oxygen, water vapor,helium, and neon gases to the mold; and passing the silica powderthrough atmosphere of the at least one type of gas supplied at the gassupplying step and then supplying the silica powder to an inner surfaceof the mold.
 37. A method of producing a silica glass crucible describedin any of claim 31 to 36, wherein the silica powder is dispersed insidethe mold, such that the silica powder is softened in atmosphere of thearc discharge prior to the silica powder reaching the inner surface ofthe mold.